A non-destructive inspection is performed to secure safety for quality of a product by checking whether or not the product has defect during the production, manufacturing or use of the product. For example, the non-destructive inspection is also performed to evaluate whether the product can be continuously used by checking a corrosion state, deterioration, whether or not a crack occurs, and the like.
A nonlinearity parameter is an ultrasonic technique capable of measuring material properties. Examples of a previously developed method for measuring the nonlinearity parameter include a capacitance measuring method and a piezoelectric receiving method. The above-mentioned methods may compute the nonlinearity parameter by measuring a displacement A1 of a fundamental frequency and a displacement A2 of a secondary harmonic wave.
Since the capacitance measuring method needs to prepare a test specimen having surface roughness of 1 to 2 μm and requires a complex calibration process for converting the measured signal into the components of a fundamental frequency and a secondary harmonic wave, it is very difficult to be used at the site.
The piezoelectric receiving method has a simpler calibration process than the capacitance measuring method, but has a problem in that a value of the computed nonlinearity parameter is varied depending on an attachment state of a probe and a kind and length of window at the time of a digital signal process.
Korean Patent Laid-Open Publication No. 10-2012-0031674 discloses a system and apparatus for measuring nonlinearity of an ultrasonic wave.